1. Field of the Invention
The present invention relates to a method of enhancing a gas-separation performance of an aromatic polyimide membrane.
More particularly, the present invention relates to an industrial method of enhancing a gas separation performance of an asymmetric aromatic polyimide membrane comprising a specific aromatic polyimide made from an aromatic tetracarboxylic acid component comprising a biphenyltetracarboxylic acid compound and/or 2,2-bis(3,4-dicarboxyphenyl)hexafluoropropane dianhydride, and an aromatic diamine component, and having specific gas separation properties, to provide an enhanced gas separation membrane having a very high ratio of a helium gas-permeation rate [P.sub.He ] to a nitrogen gas-permeation rate [P.sub.N.sub.2 ].
In the specification of the present application, a ratio of a gas permeation rate of a gas (A), [P.sub.(A) ] to a gas permeation rate of another gas (B), [P.sub.(B) ], is represented by a term, "gas separation factor [P.sub.(A) ]/[P.sub.(B) ]" of the gas separation membrane with respect to the gases (A) and (B).
2. Description of the Related Art
Various methods of enhancing a gas separation factor (selective permeation) of an asymmetric gas separation membrane having a high heat resistance have been known with respect to various polymer membranes.
For example, a known method of producing a gas separation membrane having an enhanced gas separation factor comprises the steps of preparing a gas separation porous membrane from a dope solution of a heat-resistant polymer by a wet film-forming procedure, and heat-treating the porous membrane to form a dense surface layer therein.
However, in the case where the gas separation factor of a gas separation membrane made from a heat resistant polymer is enhanced by a heat-treatment, the gas-separation membrane must be exposed to a very high temperature for a long time. This heat treatment causes the surface layer of the gas-separation membrane to be excessively and/or unevenly modified and the gas-permeability of the gas separation membrane to be reduced. Accordingly, the conventional method is disadvantageous in that the enhanced gas separation membrane having an enhanced gas separation factor cannot be produced with a satisfactory reproducibility.
Recently, various methods of enhancing the gas separation factor of gas separation membranes made of a heat resistant polymer by applying a plasma treatment to the surface of the gas separation membrane, have been proposed.
For example, British Patent Publication No. 2,089,285A and Japanese Unexamined Patent Publication No. 58-8503 disclose a method of producing a gas separation membrane (hollow fiber) having an enhanced gas separation factor by applying a plasma treatment to a porous membrane (substrate) made of a heat resistant polymer, for example, polysulfone or polyacrylonitrile, in the presence of a gaseous organic compound and/or an inert gas, to form a dense (separating) layer or to modify a surface layer thereof.
It is however very difficult for the above-mentioned plasma treatment method to form a uniform dense layer in the surface portion of the gas separation membrane or to uniformly modify the surface portion of the gas separation membrane with high reproducibility.
Also, Japanese Unexamined Patent Publication Nos. 60-99,323 through 99,327, and 62-204,825 through 204,827 disclose a method for producing a gas separation membrane provided with a dense layer by plasmapolymerizing an unsaturated fluoro compound, for example, perfluorocyclohexene, perfluoroheptene-1, tetrakis(trifluoromethyl)ethylene or tetrakis(trifluoromethyl) diethane, onto a porous membrane of a polysulfone, polyamide, or polyacrylonitrile.
Further, Japanese Unexamined Patent Publication No. 61-107,923 discloses a method of producing a selectively permeating composite membrane, comprising the steps of forming a polymer layer on an asymmetric porous membrane comprising a specific polyetherimide polymer produced from a specific 2,2-bis[4-(3,4-dicarboxyphenoxy)phenyl]propane dianhydride and a phenylene diamine by plasma-polymerizing a polymerizable monomer such as an unsaturated silane compound, and further forming a thin coating layer from an organosiloxane compound on the polymer layer.
Each of the conventional composite gas separation membrane produced by the above-mentioned plasma-polymerization method has a high helium permeation rate [P.sub.He ] of from about 2.times.10.sup.-3 to 6.8.times.10.sup.-4 cm.sup.3 /cm.sup.2.sec.cmHg. Nevertheless, these conventional composite gas separation membranes are disadvantageous for practical use in that they exhibit a low gas separation factor [P.sub.He ]/[P.sub.N2 ] of 13 to 32, and/or a low helium permeation rate of from 1.4.times.10.sup.-5 to 2.0.times.10.sup.-5 cm.sup.3 /cm.sup.2.sec.cmHg.